1. Field of the Invention
The present invention relates to image sensing apparatuses that use image sensors having photoelectric converters and driving control methods of the image sensors, and particularly relates to image sensing apparatuses that use image sensors such as CMOS sensors capable of carrying out XY addressing type scanning and driving control methods of the image sensors.
2. Description of the Related Art
In recent years there are more and more cases of CMOS sensors being used in image sensing apparatuses such as digital cameras due to reasons including that they do not require complicated timing generation circuits, are low in cost, operate on a single power source, and involve low power consumption.
CMOS sensors offer a feature not possessed by CCD image sensors in that they are capable of reading out image signals of an arbitrary region only. While having such an advantage, when CMOS sensors undergo so called rolling shutter driving, a defect not present in CCD image sensors is produced known as in-frame flicker (hereinafter simply referred to as “flicker”) since the charge accumulation timing for each line is shifted.
Here description is given concerning the flicker peculiar to CMOS sensors, which occurs when rolling shutter driving is performed. FIG. 10A and FIG. 10B are for describing a principle of flicker occurrence when a power source frequency is 50 Hz. As shown in FIG. 10A, when a fluorescent light or the like is lit by an AC power source having a frequency of 50 Hz, a quantity of light fluctuates at two times the frequency (100 Hz) of the power source frequency.
FIG. 10A also shows charge accumulation timings when imaging using a MOS-type image sensor having a charge accumulation period set to 1/30 second under a fluorescent light whose brightness fluctuates cyclically. In this case, the output signal of pixels of the first line is a value in which the quantity of incoming light from a readout point A1 to B1 is integrated. Similarly, the output signal of pixels of the second line is a value in which the quantity of incoming light from a readout point A2 to B2 is integrated. Thereafter, the same is true until the final line. At this time, since the phase of the charge accumulation period with respect to the cycle in which the quantity of light of illumination varies for each line, the integrated values of the quantity of incoming light vary, and luminance fluctuation (flicker) appears in the frames. When the image sensor is constituted by 480 lines, the time required for reading out a single line is 1/14,400 seconds (= 1/30÷480 lines), and therefore phases of accumulation commencement timings match at cycles of 144 lines with respect to changes in the quantity of light of illumination. For this reason, the flicker in this case appears as fluctuation in the luminance level for cycles of 144 lines (FIG. 10B). When a CMOS sensor undergoes rolling shutter driving in this manner, luminance disparities occur in the frames.
To address these problems, Japanese Patent Laid-Open No. 2003-189172 discloses that when the power source frequency is 50 Hz, the shutter speed is set to n/100 seconds and when the power source frequency is 60 Hz, the shutter speed (charge accumulation period) is set to n/120 seconds. By controlling the shutter speed in this manner, the integrated values of the quantity of incoming light per line become equivalent and in-frame flicker can be prevented.
On the other hand, CMOS sensors also carry out readout control of image signals of only the required region when performing electronic zoom, which makes use of the CMOS characteristic of being able to perform readout of image signals of only an arbitrary region. For example, Japanese Patent Laid-Open No. 2002-314868 discloses that electronic zoom of a higher resolution is carried out for a wider zoom range by changing the readout region and driving method of an image sensor of an XY addressing type scanning method.
However, the following problems occur when changing the readout region or driving method of a CMOS sensor as described in Japanese Patent Laid-Open No. 2002-314868 while performing rolling shutter driving of the CMOS sensor in a state in which the shutter speed is fixed at n/100 seconds for example as described in Japanese Patent Laid-Open No. 2003-189172.
FIG. 11 shows charge accumulation timings and readout timings in a case where the driving method is changed and the readout period changes while capturing images or moving pictures or the like for an electronic viewfinder (EVF). It should be noted that in FIG. 11 the power source frequency is 50 Hz and the charge accumulation period is n/100 seconds. Furthermore, the timing for changing the driving method is set to during a vertical blanking period after the readout of one frame of image signals has been completed upon receiving an instruction to alter the driving method.
In FIG. 11, the driving method of the CMOS sensor is changed at a time t1 from an image sensing driving method A whose readout period is long to an image sensing driving method B, which has a readout period faster than that of the image sensing driving method A. In this case, the reset commencement time of frame 3 is before the time t1, and therefore in the period before the time t1, the reset period is the same as the readout period of the image sensing driving method A. However, since there is a switch to the image sensing driving method B from the time t1 onward, the resetting is carried out with the same period as the readout period in the image sensing driving method B. A result of the reset period change before and after the time t1 is that disparities occur in the charge accumulation periods at the top and bottom of the screen in frame 3 as is evident from FIG. 11.
Accordingly, it is conceivable to perform control such that brightness disparities at the top and bottom of the screen do not occur by setting the reset period in frame 3 to the same as the readout period after the switch in image sensing driving as shown by the bold lines in FIG. 11. However, a result of matching the reset period and the readout period in this manner is that the charge accumulation period is not n/100 seconds, and flicker occurs in frame 3.